Wide band probe



Oct. 4, 1949. D. D. GRn-:G ETAL,

WIDE BAND PROBE 4Filed Oct. 30. 1945 l kbl 9km ok x :i z j Patented Oct. 4, 194g UNITED Sfar-E1s Peris-nr" oel-Tien;

WIDE 'BAND PROBE Donald .Dr Grieg and. Arnold-M.: Levine, Forest` Hills, and Lester Dubin, Brooklyn, N. ,Y. aS.- signors to FederalA Telephone and Radio Corporation, New York, N. Y.,`a corporation vof Y Delaware ApplicationOctober- 30, 1945, Serial.No..625,64-8

low input `capacity .and at the .same time ,is .proj ductive of an overallzloss many times less than that of the probesusedin the past.

It another. object to provide aprobewhich makes, use .of two cathode .followers in tandem,

Still .another object is toprQVde Va DIQbe Which isthe cause of a.aru: u11.t,ofgloadin-g` on thecircuits under test..

A still further object is3to provide va probe which, is.. operative over an-,eXtre1r1`1yWde frequently: band down to and includingdirect current with.-` out being. the causel of deleterious` characteristics dueto Vthe probe itself.v

A further object is to provideygaprobewhichV has A`asubstantially flatsubstantially constant time. delay characteristic over a wide band of frequencies. including direct current.

In accordance with certain features of the invention a probe has beenrprovided which offers a lowinput capacity .whichV at the same time feeds a low impedance outputwcableL These requirements have been met by using two cathodey followers ini-tandem. Theinputcathode follower circuit-is of such a characterr` as to offer` ayery high inputimpedance serving the dual,function of,meeting phase shift requirements and also reducing the effective input capacity. Theoutput cathode follower is coupled ldirectly tothe rst stage and is arranged to feed a low impedance coaxial cable. A selector switch in conjunction with theinput stage makes itpessible to feed the 3 Claims. (Cl. 179-171) the acCQmDanyine drawings which. thesinele ligure represents a circuit diagram in schematic form of the probe embodying our invention.

Referringnowvto, the drawing, the input to A the probe is. appliedover an 11.191.111k lead I whichv` is provided with a probe. tip? and which byA Way of a .double-throw double-pole .sw-itch y3 applies-v the input voltage over a couplingcondenser 4 and aigrid return resistance 5 to a gridA 6.0i a;

triode 'I. If desiredthe condenser 4 maybe bypassed, as indicated at 4a when-direct current voltage is to be applied. The triode 1, further including a plate anda-cathode 9,'is energized from a plate voltage supply B+ at IIJ andv is directly'coupled to a grid I I of an output cathode-follower triode I 2` fr omits cathode circuit by vWay ofa connection4 I3., The cathode circuit. of the triode 1 includes-.in -its connection to a ground lead I4 ,a bias and load resistor I5 as 1 wellas an `additional load resistance I6, the latter being comparatively large. The output triode l2 which includes aplate I'I and a cathode I8 receives-its plate energizing-.voltage from the plate potential vsupply. available at I0. over a lead. I9, which includes a phase correction circuit 2E]y comprisedof avvariable .resistor 2I and a capacitance 22. The-output of thel probe is obtainableA fromthe circuit ofthe cathode I8 over ashielded coaxial cab1e23 and vmay be `appliedto an vindicator such as a cathode, ray oscillograph'` over a.

connectionl 24. Thezoutput-circuit of thecathode L8.` includes` a resistancel 5A ,formingsa matching.

provided. from the shielded cable 23 to the ground lead I4.. by .way of aconnection 30, TheA input to,

the triode- -1 maybe applieddirectlyby wayof a jumper connection 3L whenthe switch 3. makes contact with terminals32 and. 33.y When the switch 3 iscOntaCting. terminals 34 and. 35 a. capacitance compensated attenuator network 36j.

is -broughtinto circuit withv the inputftothe triode 'I. The attenuator network comprises two resistances 31 and 38 the former in series with the input lead, the latter forming a connection to ground therefrom. These resistances are shunted by capacitance 39 and 40, respectively, the former being adjustable. Contact 35 forms the output terminal for the attenuator, over a connection 4|. The attenuator network 36 is also of the embodiment to be read in connection with 55 connected to the ground lead I4 over a connec-v tion 42. The probe circuit is completely shielded as indicated by the broken lines at 43.

In operation it has been found that a comparatively large value for the cathode load resistances I5-l6 should be chosen so as to provide a stage gain close to unity with a consequent reduction of the input capacity and a high effective input impedance. However, several limiting factors as to the size of the cathode load resistance have to be considered, the following stage being directly coupled. Thus, since the grid bias of the output stage depends on the voltage drop across the cathode load resistance I6, too large a value therefor would cause the drawing of large grid current and overloading of the input stage. It is also to be noted in connection therewith that the band width characteristic of a cathode follower is determined by a factor which is dependent on the ratio of the grid bias to the input voltage of the rst stage. To obtain a minimum of phase shift at low frequencies the largest possible value of input coupling capacitator 4 is desired. Since the probe as a whole must offer a low input capacity the selection of an input cathode follower will offer a very high input impedance serving to meet phase shift requirements as well as reducing the effective input capacity. The direct coupling of the output cathode follower to the first stage avoids the use of frequency dependent coupling elements.

The input attenuator is a capacity compensated resistance divider network of a given ratio which is effective independent of frequency. The parameters of the attenuating network have been made such that the ratio of the capacitances is equal to that of the resistances insuring a Zero phase shift and equal attenuation for all frequencies. The output cathode follower feeds a coaxial cable 23 having a given characteristic impedance which is terminated with matched impedance given by the resistance 25 in order to minimize reflections at high frequencies.l This provides for a matching substantially equal to the impedance of the cable for all frequencies whereby reflections will be minimized.

In view of the fact that a large biasing resistance (2T, 28) is necessary for the cathode i8 because of the direct coupling of the two cathode followers, the resistors 21, 28 are provided with a high frequency by-pass in the form of the capacitance 29, in order for the terminating impedance 25 to be effective at high frequencies. The phase shift introduced by this combination at low frequencies is substantially compensated by the time delay or phase correction network 29 in the plate circuit of the triode I1. In order to obtain a substantially flat overall time delay characteristic for the probe over a wide range of frequencies down to direct current, a suitable relationship is used wherein the ratio of the resistances in the networks 2D and 26 equals the ratio of the capacitances. Any necessary adjustment with the varying frequencies may be made by means of the two adjustable resistances 28 and 2|.

While we have described above the principles of our invention in connection with specic apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of our invention as defined in the objects and the accompanying claims.

We claim:

l. A wide band voltage probe, comprising a pair of cathode follower tubes in tandem, probing means, input control means disposed between said probing means and the first of said cathode followers, output circuit means for the second of said cathode followers, and phase correction network means operatively associated with said second follower comprising a bias resistor network for the cathode Iof said second follower compensated by capacitance and a time delay network including resistance and capacitance in the plate circuit .of said second follower, the values of said resistances and capacitances of the said two correction networks being proportioned to make the ratio of the resistances of the two networks equal the ratio of the capacitances whereby a substantially constant overall time delay characteristic for the probe may be obtained over a wide frequency band including direct current.

2. A Wide band voltage probe, comprising a pair of directly coupled cathode follower tubes in'tandem, probing means, input control means associated with said probing means, a coupling network intermediate said input control and the rst of said cathode followers, means for increasing the input impedance of said first follower, output circuit means for said second follower, matched terminating impedance means for said output circuit, high frequency )oy-passed biasing means for the cathode of said second follower in series with said ,terminating impedance, and a time delay compensating network in the plate circuit of said second follower to compensate for phase shift in the cathode circuit thereof, whereby a substantially flat overall time delay characteristic may be obtained over the frequency band used.

3. A probe according to claim 2 wherein said compensating network includes a phase shift adjustment means.

DONALD D. GRIEG. ARNOLD M. LEVINE. LESTER DUBIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,613,422 Wegel Jan. 4, 1927 1,943,986 Nyman Jan. 16, 1934 2,185,870 Smith Jan. 2, 1940 2,252,613 Bingley Aug. 12, 1941 FOREIGN PATENTS Number Country Date 463,370 Great Britain Mar. 30, 1937 

